The invention relates to a method of controlling a stepping motor where, after the starting of the motor by a motor advance pulse, feedback pulses depending on the motor position are used for controlling the motor, as well as to an arrangement for carrying out this process.
Electric stepping motors have a wide field of application in all those areas where precisely specified distances of different lengths have to be covered. Stepping motors are, for instance, used in data processing systems for advancing form sheets in printers, furthermore for driving transport devices of write or read heads in storage devices, and as positioning motors in control systems. A characteristic feature of these stepping motors is that they can be controlled by advance pulses, and that they can perform a discretionary number of step motions upon each applied advance pulse. Similarly to the synchronous motors, stepping motors are designed with distinct poles and execute upon each advance pulse applied a movement which corresponds to a pole pitch.
Two basic methods are known for controlling the stepping motors. In the one method the advance pulses have to be applied to the motor with constant frequency, independently of the motor speed reached or the existing load conditions. In order to make sure that each individual motor advance pulse is converted into an actual movement of the motor by one pole pitch, the frequency has to be low, particularly when starting the motor, for otherwise the individual advance pulses would not be converted into step motions. Consequently, however, the speed of the stepping motor which can be reached is low and the motor is not suitable to be used with high speeds. Such a manner of control is, for instance, described in German Auslegeschrift 1,223,039.
This known German Auslegeschrift also describes the basically different method where on the shaft of the motor a time disk is arranged which according to the movement of the motor emits, upon each individual advance pulse and the corresponding movement by one pole pitch, a feedback pulse. In this known arrangement, this time disk pulse is used for reading out the advance pulses from a pulse storage and for applying them to the driving circuit and the next motor coil. Therefore, in this arrangement known from DAS 1,223,039, the feedback pulse mode is employed where a new motor advance pulse is generated directly from each time disk pulse. This manner of controlling a stepping motor has the considerable advantage that the motor advance pulses can be applied to the motor, when owing to its magnet wheel position reached, the motor can convert a new advance pulse into another step motion. Thus, different load conditions are taken into consideration and the motor accelerates itself. Compared with the open loop mode where no feedback or time disk pulses, respectively, directly generate or excite the motor advance pulses, the starting phase is thus considerably reduced and a much higher final speed is made possible.
For reaching higher final speeds it is for instance known from DAS 2,119,352, to insert, for accelerating the feedback mode operated motor from a lower to a higher speed, once only, between two motor advance pulses an additional pulse as a motor advance pulse, so that the motor reaches a higher final speed. It is furthermore described as known in this DAS that different final motor speeds can be reached in that the lead angle is altered in steps, according to the speed obtained.
For the stepwise conversion of the lead angles, the time disk provided on the motor shaft shows slots differently spaced from the circumference, said slots activating photodetectors, according to different lead angles. The terms lead angle or phase angle relate to the electric angle existing between the supply voltage applied externally to the coils and the internal electromotive force, or the induced magnet wheel voltage, respectively.
For decelerating the stepping motor from higher speed ranges it is known that, as described for example in DAS 2,119,352, the additional acceleration pulse is blanked out again during the delay, i.e., that a feedback pulse is not used for generating an advance pulse for the motor. Thus, the rotary field of the motor precedes the externally applied rotary field so that the delay is thereby achieved. In German Patent Application P 22 49 757.0 it has been suggested that two feedback pulses in the delay phase are not used for forming motor advance pulses. Thus, the motor is decelerated much more quickly from the high speed to lower speeds. In order to ensure a safe deceleration into standstill it has been suggested there that upon reaching a predetermined speed which can be determined owing to the repetition frequency of the feedback pulses, an additional pulse is applied as motor advance pulse, i.e., a so-called hold pulse, keeping the motor on a predetermined lower speed. From this lower speed phase, the motor is then slowed down to standstill by means of stop pulses in order to reach the precise predetermined target point.
This following mode with a lower speed is shown in FIG. 2 in curves 27 and 28 and it has the disadvantage that, according to the load conditions of the motor, the latter comes to a standstill more or less quickly. The necessary overall time required for reaching the desired position is thus increased compared with the optimum time.
As generally known, the torque of the stepping motor is one of the parameters which gives maximum information on the quality of the motor. This torque depends to a high degree on the motor current and phase angle .theta.. As specified, phase angle .theta. is that electric angle which exists between the voltage applied to the motor coils and the magnet wheel voltage induced by the motor. A desirable feature for each stepping motor is that the torque as a function of the speed or, in other words, of the stepping frequency f.sub.S, is as constant as possible and always as high as possible. It is a known fact, however, that the torque decreases with rising frequency owing to the voltage drop, and the torque furthermore depends considerably on the amount of the phase angle. When the phase angle is fixedly determined, as in the ordinary closed loop mode, the phase angle is mechanically fixedly determined by the preceding of the feedback pulses relative to the motor advance pulses, or additionally adjustable by 90.degree. by inserting an acceleration pulse, as shown in curve 29 of FIG. 3 -- the torque in most cases is very far from its optimum value. An adaptation of the phase angle in individual steps does not permit, either, the necessary optimization of the phase angle as a function of each individual speed reached.